Electricity Lecture Series Charges & Charging Applied Sciences Education Research Group (ASERG) Faculty of Applied Sciences Universiti Teknologi MARA email: drjjlanita@hotmail.com; drjjlanita@yahoo.com http://www3.uitm.edu.my/staff/drjj/ 0193551621 Electric Charges At the end of this unit, you will be able to: 1. Explain the gravitational forces acting on any object. 2. Mathematically represent the gravitational force and describe its impact on physical events. 3. Describe existence of electrical charges in matter its magnitude, mass and its quantization property. 4. Sketch and explain the charging by induction and charging by contact numerically and diagrammatically. Alam, Malaysia 1
GRAVITATIONAL FORCES Galileo Science: All objects regardless of size, shape or mass will fall at the same rate Newton extended the principle: Universal Gravitational Law: All object will attract each other with force inversely proportional to square of distance 21 F m m r 2 1 2 F 21 = G m 2 1 2 r m ATOMIC STRUCTURE Alam, Malaysia 2
Electric Charges Matter: made up of atoms and molecules Atom: made up of nucleus, protons and electrons Charged object: imbalance number of electrons & protons Positively charged Negatively charged Conductors: charges can move freely Insulators: charges cannot move freely Electric Charges Matter: made up of atoms and molecules Atom: made up of nucleus, protons and electrons Charged object: imbalance number of electrons & protons Positively charged: ve<+ve Negatively charged: ve>+ve Conductors: charges can move freely Insulators: charges cannot move freely Alam, Malaysia 3
18.1 The Origin of Electricity Cutnell & Johnson 7E The electrical nature of matter is inherent in atomic structure. m p m n = 1.673 10 = 1.675 10 27 27 kg kg m e = 9.11 10 31 kg e = 1.60 10 19 C coulombs Electric Charges Charge quantization: charges exist in multiples of an elementary charge, the charge of an electron q = Ne = e, 2 e,.. where N are the number of electrons & the elementary charge e is e =1.6 x 10-19 C Number of charges in 1 C?? N=q/e =1 C/1.6 x 10-19 C N = 6.25 x 10 18 N 1 2 5 10 Q (x10-19 C) 1.6=e 3.2=2e 80=5e 16=10e Alam, Malaysia 4
Charges, charging, electrical force & discharging Matter Conductor Insulator Atom Charges Electron Proton Positive Negative Attract Repel Neutral Charged Discharged Conduction Induction Friction Contact Ground Lightning Force distance Highest electron affinity Rubbing wool to rubber caused rubber to have excess electrons which were transferred from rubber Charging by Friction Alam, Malaysia 5
Charges, charging, electrical force & discharging Charging by contact Bringing the rod near the pithball causes polarization (separation of charges) Alam, Malaysia 6
Charging by contact Bringing the rod near the pithball causes polarization (separation of charges). Charging by contact Bringing the rod near the pithball causes polarization (separation of charges). Touching the rod will allow electrons to flow to the rod. The rod remains positively charged since the number of electrons transferred is far too small to neutralize the positive charges Alam, Malaysia 7
Charging by contact The pithball is now repelled since it is positively charged after losing electrons to the rod via contact Charging by contact When the rod is pulled further away, the charges on the pithball redistributes evenly. The repulsion between the rod and ball is smaller because the rod is far away. Alam, Malaysia 8
Charging by contact The pithball is now neutralized by grounding (pathway to transfer electrons to the positively charged pithball) it with my finger. Charging by contact Pithball is polarized (separation of charges) when the rod is brought nearer. The electron on the pithball is being repelled by the negatively charged rod. Alam, Malaysia 9
Charging by contact Pithball is polarized (separation of charges) even more when the rod is brought nearer. The electrons on the pithball are being repelled by the negatively charged rod. Charging by contact Pithball is polarized (separation of charges) even more when the rod is brought nearer. The electrons on the pithball are being repelled by the negatively charged rod. Alam, Malaysia 10
Charging by contact Electrons move from the rod to the side of the pithball which is being touched making the pithball has excess electrons. The rod remains negatively charged because it only lost a small number of electrons Charging by contact Since the rod and the pithball are both negatively charged, the pithball is being repelled strongly. Alam, Malaysia 11
Charging by contact The repulsion is getting smaller when the rod I pulled farther away. At the same time, the electrons on the pithball begin to distribute evenly throughout the ball. Charging by contact The ball is being grounded (leaking off the electrons to earth ie finger) to neutralize the pithball. Alam, Malaysia 12
18.4 Charging by Contact Cutnell & Johnson 7E Electrons are transferred to the neutral conducting sphere when the sphere is touched by the negatively charged rod. Charging by contact. 18.4 Charging by Induction Cutnell & Johnson 7E Charging by induction is a 3-stage process: 1. Bring a charged rod near the sphere to cause polarization of the charges Charging by induction. 2. Ground the side of the sphere which is furthest from the charging source. 3. Remove the charging source Alam, Malaysia 13
Charging by Induction: 1. Bring negatively charged rod near the sphere 2. Ground the sphere to remove the electrons 3. Sphere is positively charged Animation source from: The Multimedia Physics Studio website and The PhET website Charging by Induction: Two Neutral conducting spheres 1. Bring negatively charged balloons near the sphere 2. Pull the second sphere after electrons have migrated to the second sphere. 3. Sphere 1 is positively charged and sphere 2 is negatively charged Alam, Malaysia 14
18.2 Charged Objects and the Electric Force Cutnell & Johnson 7E LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved). Total number of negative charges (electrons) and positive charges (protons) must be equal Consider the fur and rod together as a system. Since the system is uncharged initially, then the total charge must be zero before and after rubbing. Hence if rod acquires 6e due to rubbing (friction), then the fur must have lost 6e, the total charge for the fur-rod is zero. 18.2 Charged Objects and the Electric Force Cutnell & Johnson 7E LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved). Total number of negative charges (electrons) and positive charges (protons) must be equal Alam, Malaysia 15
18.2 Charged Objects and the Electric Force Cutnell & Johnson 7E LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved). Total number of negative charges (electrons) and positive charges (protons) must be equal Charge Conservation A 5q B -3q C 3q Shown are conducting spheres each of charges 5q, -3q and 5q What is the total charge on the spheres? Sphere A touches sphere B and then separated. What is the total charge after the process above, the charge on each individual sphere? A 5q B -3q A 2q B 0q A q B q Alam, Malaysia 16
Charge Conservation A q B q C 3q Shown are conducting spheres each of charges 5q, -3q and 5q What is the total charge on the spheres? Sphere B touches sphere C and then separated. What is the total charge after the process above, the charge on each individual sphere? B q C 3q B 2q C 2q Alam, Malaysia 17